Internal combustion engines which burn pressurized gaseous fuel have long been used to power such devices as fork lifts operating inside buildings. Conventional fuel systems for such engines provide an enclosed gas chamber that has one movable side, i.e., a diaphragm, with the chamber being replenished by controlling a fuel valve to provide a fuel source that has a pressure slightly below atmospheric. There is communication provided between this chamber and the throat of a venturi in the induction system, with an orifice between the chamber and the venturi throat to control the fuel flow. The diaphragm is usually physically connected to the fuel valve through a lever. These systems require idle progression systems, and it has been found that conventional fuel systems do a less than optimal job of matching fuel supply to demand through the transition from idle to higher fuel demand conditions. The conventional fuel system also requires a starting assist system such as a choke or electric primer solenoid.
Another prior art gaseous fuel charge forming device uses opposing mechanically connected diaphragms of different areas with a fuel valve on the outer side of the smaller diaphragm. Negative pressure signals from a venturi are applied to a point between the two diaphragms, which unbalance the diaphragms in the direction of the smaller diaphragm to open a fuel valve by way of a lever. Fuel then flows across an orifice and thereby causes a back pressure to balance the diaphragm forces. This dual diaphragm prior art system is a positive feed device, such that fuel can be fed to the engine at a point upstream of the throttle valve other than the venturi throat. This dual mechanically-connected diaphragm fuel system requires an idle progression system adapted to the specific engine with which the system is used as well a starting system as in the single diaphragm system described above. This system is also expensive to build, due to the complication of the dual diaphragms and lever/valve arrangement, as well as the relatively large chamber (approximately 4.5" in diameter) required for housing the large diaphragm.